Telephone system



H. H. SCHROEPPEL TELEPHONE SYSTEM Filed Deo. l5, 1943 A Jan. 9, 1945.

Patented Jan. 9, 1945 `UNifTE/Dv STAT s mr ENT orrlfCiE A 'rELErHoNE SYSTEMy Harold Henry Schroeppel, Glen Rock,VY N. J., as-

signor Ato American Telephone and Telegraph Company, a corporation of New York" f. vmmlimtion Dem'ber 15, 1943,vt sei-iai No.'514,3o9',

' 3 Ciaims. (01.'179-18) This invention'relates to telephone systems and more particularly to telephone systemsv of the panel type.KVV

In panel telephone systems it sometime-s happens that aftera connection between ka-calling party and acalled party has. been completed a seekaconnection toone of the 'busy parties, such as the calling party. Sucha condition will arise Where the calling party has one or more assigned lines but the traflic tothe calling party is ,sometimes muchA heavier than the availablelines can carry. Under `such'conditions the central office switching apparatus may-act' erroneously to connect somelor all of the additional calling parties to a diiierent .line assigned to a'party other than the one called. This will cause considerabler annoyance not only tov the calling parties who have been given the wrong number, but also' to the party orv parties who may receive such calls'in error. It is an object of this invention'to' overcome the type of diiculty-justreferred toby providing improved apparatus to return a busy signal to a considerablenumber of calling partiesI who practically ysimultaneously endeavor to reach a lousy party. f

The central office switching failure responsiblefor the undesirable conditionslabove referred to arises from the simultaneous connection of the relays of a plurality kof nal selectors to asingle sleeve terminal of thenal frameassociated with the busy partys line.' The 'relay circuits ofthe diierent final selectors are connected effectively in parallel with each other from the same sleeve terminalr andthe parallel 'paths ofv these final selector relays receive reduced current from a common central office battery through 'a resistor of predetermined magnitude. If the' relaysy of one final selector are connected to thenalframe t sleeve terminal the voltage available to the relays *e of but one nal selector will be suflicient to satis-v factorily control'their operation. When therelaysl of several final selectors are bridged to the same sleeve terminal, however, the available voltage for controllingtheoperation of the groups of nal selector. relayswill be insucient for'thispurpose. Consequently the final selector relays will fail to operate properly. The up-drive mechanism controlled 4by these vnal selectors will therefore continue to hunt to the next line at the i number of additionalpartieswill simultaneously.

vThe objects and features of this invention will be more readily understood from the more detailed description hereinafter following when read in connection withtheaccompanying drawing in which Figure 1 shows the invention generally applied to part 'of the centralv oice equipment of a panel type of telephone system, and Fig. 2` shows in simplied form only the parts 0f the central oiceequipment comprisingthe invention...r i

final frame to establish a telephoneconnection between that next line andthe various calling` parties. As the party connected to thenext line' is different from theV one called by the variousy Fig, klishows `as much of the central oce equipment as is required to explainv -the invention. Here one of anumber of subscribers having the telephonel subscribers set SSi desires to call the yparty having the subscriberss'etrSSz.- When the calling party removes the 'receiver from the switchhook of the subsetSSi,v a line -relay L willv be operated over a kcircuit e-Xtendingfronfi ground on theright armature andthe back contact of the cut-ofi relay CO through equipment including theT terminal of the line finder terminal strip LFTS, the vertical side of the intermediate distributing frame VIDE', the horizontal side of ther intermediate distributing frame `IIIDF, the main distributing frame MDF,` through the subset SSi, then through equipment including the i ring terminals R of the main distributing frame MDF', the horizontal side of the intermediate distributing frame HIDF, the vertical side of the intermediate distributing frame VIDF, the line lfinder terminal strip LFTS,` then over the back contact and left armature of the cut-off relay CO, through the winding of line relay L, to grounded' battery B1'. The operation of the line relay. L serves'toY operate the line finder start and trip mechanism LFSC by the completion of yan obvious circuit extending through the left armature and make contact of line relay L.A'The line nder mechanismLFSC starts the operation of appa'- ratus to cause ther line nder district selector circuit LFDSassociated with the'samer line nder mechanism LFSC to hunt for the calling line to which the subset SS1 4isconnected. After the calling line is located, an idle trunk is found to connect the subset SSi 0f the calling party to the subset SSz of the called party through the terminating portion of the panel central office equipment (not shown) It will be observed that the subset SSz of the called party is connected through the line finderdistrict selector circuit LFDS to the tip and ring terminals T and R 0f the line finder bank LFB and that these same terminals extend to the calling partys subset SSi. It will also be observed that the line finder district selector LFDS connects a central oiiice battery Bo through resistor Ru and through the sleeve terminal S of the line nder bank LFB to the sleeve terminalSL ,of the v line nder terminal strip LFTS. The resistance Ro is the eiective impedance in series with the become operated. The mechanism of circuit SHC will be more fully described with reference to Fig. 2. The operation of relay CO will open the circuit to line relay L at the back contacts of the right and leftarmatures of relay CO, whereupon relay L will release. VThe release of relay VL will release the line finder trip and start mechanism LFSC by opening the circuit 'thereto at the make contacts of the right and left armatures of line relay L.

'It will be observed that when line relay L operates, it connects aground to the line finder trip and start circuit LFSC through its left armature to cause the district selector LFDS tostart moving up .to nd the calling line as already noted. Line relay L also connects battery through a resistance (not shown) in .the line all) finderk start and trip .circuit LFSC to the hunt terminal Hof the line nder bank LFB. When thev H brush of `district selector kLFDS reaches terminal H, the battery from the line iinder .trip circuit described `above causes `the relay HH in the districtselector LFDS to operate, stopping the up-drive. The district selector LFDS then applies battery Bq through resistance Ro through the lcircuit mechanism ISHQ to the winding of the cut-olf relay CO, causing it to vbecome operated. Although the circuit and mechanism designated SHC is shown in series with the cut-oil? `relay CQ, no s uch circuit or mechanism was used heretofore, the sleeve contact SL of the terminal strip LFTS being directly connected to the winding of relay CO.

It is noted that the tip, ring and sleeve contacts T, R and S,`respectively, of the horizontal side of the intermediate distributing frame HIDF are connected to the multiple contacts T, R and S of the nal frame FF associated with the calling line. When the line nder district selector LFDS reaches the position shown in the drawing, the

battery Bo will also be connected through resistor f' set ssi.

another subscribers set such as SSz, then when another party (not shown) dials the number corresponding to subset SSi, the brush BR1 of the iinal selector FSi associated with the new calling party will engage sleeve contact -S of the final frame FF as shown in Fig. l. The relay PBXi of the final selector FSi will become operated and cause the nal .selectorFSi to actuate mechanism to return a busy signal t0 the calling party. The operation of relay PBXi also stops the up-drive mechanism (not shown) of the nal selector FSi and thereby prevents the connection of the additional calling party to any other line.

lIt will be seen that when the subset SSi is connected to the subset SSz as shown in the drawing, the grounded battery Bo is connected to the sleeve terminal SL through resistor Ro. The sleeve terminal -SL is in turn connected to ground over two -parallel paths, one of which inclu-des the winding of relay CO and the other the windings of relays TBi and PBXi vof the final selector FSi. The impedances of the. two parallel paths are about the same (perhaps 1000 ohms each in existing installations) and the portion of the voltage of battery Bo applied to the winding of relay PBXi will be suflcient to operate relay PBXi to -stop the up-dri-Ve mechanism of final selector FSi.

The final selectorFSi will therefore sto-p hunting and not overstepr to ther next subscribers line.

However, if more and more parties are simultaneously calling the same' party having the subset SSi, the brushes BH2, BB3, etc., of the final selectors FSL, FSs, etc., corresponding to these additional parties will all engageappropriate sleeve contacts S of the final frame FF associated with the line of the subset SSi. Consequently additional final selector relay circuits such as 'FB2- PBXz, TBa-r-PBXS, etc., will simultaneously be paralleledl to ground vfrom `sleeve contact SL (along with the previous Apath established. to ground by relay CO). In prior art systems in which .the circuit and mechanism SHC was not present, the additional parallel paths reduced the voltage `from battery Bo applied to the windings of vrelays PBXz, PBXa, etc., to `,such a low value that these relays failed to operate. The lfailure to operate these relays caused the up-drive mechanisrn of the corresponding unal selectors to overstep to the next subscribers line (not shown). Thesubset connected to the latter line was therefore rung in error. This situation produced confusion not alone `for the Ypartylor parties rung in error, but kalso for the additional parties seeking tocommunicate with the busy party having sub- The prevention of overstepping of final selectors simultaneously vtesting. a busy line is a primary feature of this invention and it is accomplished by inserting .the ,apparatus SHC in the path between sleeve terminal SL of the line finder terminal strip LFIS and the winding of relay CO.

One fo-rm of circuit arrangementwhich is suitable for preventing or greatly reducing the occasions for overstepping on iinal selectors when they simultaneously reach a busy line, is shown in Fig. 2. Here the sleeve terminal VSL of the.

line finder terminal strip LETS of'Fig. l is shown connected to two equal coils Piland P2 of relay DS which is of the differentia-ltype. Coils Pi and Pz of relayDS are in turn connected to the wind- .ingsPi and P2, respectively, of the polar relay OF.

The upper windingPi of relay OF is connected through a resistor Ri to ground., yThe lower winding P2 of `relay OF is connectedto the lower make contact of relay LSwhiclr is ofthe slow release zupper winding P1.

" station such as SS1, the line finder ldistrict selector LFDS will connect battery Bov through resistorRo tothe sleeve' terminal SL las already explained. Current from battery Bo will then iiow through vresistor Ro, through Winding P1 of relay DS, through winding P1 of relay OF, and through resistor R1 to ground. No current will traverse the lower windings P2 of relays DS'and OF because the circuit from battery Bois open at the lower make contact of relay LS. However, relay DS will become operated'by lthe current owing through its upperwinding P1.` The operation of relay DS will cause relay LS to operate, the operating circuit including battery B2, the winding of relay LS, and the make contact` and upper armature of relayDS.Y The operation of relay LS will cause 'the cut-oir relay CO to operate over a circuit including battery B2, the upper make contact and armature of relay LS, the winding of relay CO and ground. The operation of relay CO will perform the functions already described in regard to Fig. 1. The upper winding P1 of relay OF is a biasing winding and the flow of current from battery Bo through this winding is in such a direction that relay OF will remain unoperated. The registering mechanism RC will not operate due to the flow of-current from battery Bo through winding P1 of relay OF.

The operation of relay LS already described connects battery B2 to the sleeve terminal SL over a circuit including the lower armature and make contact of relay LS and the lower windings P2 of relays OF and DS. These windings are of low resistance and their combined resistance is considerably smallerthan the resistance R1. The addition of the auxiliary battery B2 to sleeve terminal SL is made through the resistance of the lower windings Pz of relays DS and OF and this resistance-which may be, for example, 100 ohms or less-is considerably less than the resistance Ro in series with battery Bo. Thus, batteries Bo and B2, which may have the same terminal voltage, are both connected to sleeve terminal SL at the same time. Moreover, the reduced resistance in series with battery B2 will raise the potential of sleeve terminal SL and Permit the connection of an increased number of final selector circuits such as FS1, FSz, etc., to terminal SL without causing these iinal selectors to overstep. Hence these final selectors will stop hunting When they reach the desired busy line.

l After relay LS Voperates to connect battery Bz to the sleeve SL, battery B2 will send current through both windings P1 and P2 of relay. DS over a circuit including battery B2, the lower armature and make contact of relay LS, the lower windings P2 of relays OF and DS, the upper windings P1 of relays DS and OF, resistor R1 and ground. The effect of the' current of battery B2 on windings P1 and P2 of relay DS is such as to produce equal and opposing magnetic elds in the core of this relay and hence battery B2 will not affect relay DS. The operating current for relay DS will ordinarily emanate from battery Bo, 'especially when only one subscribers set is connected to the set SS1, and this current will traverse the Relay DS should be designed to'remain operated after relay LS operates even though the auxiliary battery B2 is connected differentially with respect to the windings P1 and Pz of relay DS, n I

Whenadditional final selectors suchas FS1, FS2, etc., are connected to the circuit as shown, the current traversing the lower windings P2 of relays DS 'and OF will increase and exceed the current traversing the yupper windings P1 of these relays.. The magnitude vof the current throughl thelowerwindings P2 Aof relays DS and OF will be proportional tothe number of i-lnal selector circuits simultaneously connected to the sleeve terminal SL. The magnitude of the current through the upper windings P1 of relays DS and OF` will decrease, land this current will be pro'- gressively smaller than that through the lower windings P2 as more final selectors are added to the. circuit. The relatively larger current `flow through winding P2 of relay OF will be suirlcient to overcome `the biasing effect ofl the current through its upper winding P1, whereupon relay OF will operate. ,v The operation ofv relay OF will cause the register RC to operate and thereby in-v dicate that additional parties are attempting to reach the same partysimultaneously. The register RC will therefore provide a rough record of the number of times this overloaded condition of the circuit occurs.

In this regard it is pointed out that the relatively increased current through the lower winding P2 of relay DS may also cause relay DS to release momentarily at the instant of change even while the subscribers sets SS1 and SSz are connected to each other. However, the relay LS is of the slow release type and it will not release during this brief interval.- I y When theA line finder district selector circuit LFDS disconnects from the line extending to the Isubscribers set SS1 upon the completion of the call, the relay DS of Fig. 2 will release to release the relays LS and CO. Although relay LS has been described as a slow-release relay, its delay constant should be long'enough to permit relay DS to reoperate upon the simultaneous connection of a number of final selectors to the circuit, as already described. But the relay DS should not be so slow as not to respond to the disconnection of the district selector LFDS. If relay LS were too slow, the auxiliary battery B2 would remain connected to the sleeve SL and hence final selectors such as FS1,-F52, etc., would register busy v.conditions even when the subscribers set SS1 is idle.

The relay OF and the register mechanism RC controlled by it are required only for recording purposes as already mentioned. If no record of the busy condition is required, the relay OF and mechanism RC may be omitted if so desired.

The arrangement shown and described hereinl above has been found to operate satisfactorily in panel systems in which the cut-off relay CO has one terminal of its winding grounded as shown. It'will be understood that the general principles of thisinvention may be applied to other and widely varied organizations without departing from the rspirit of the invention and the scope of the appended claims.

What is claimed is:

1. In a telephone system, the combinationof a .called partyfs telephone seit, :said Ameans 'including means 'for `simultaneously connecting "saidzfirst source of voltage rbc .-the multiple -:sleeve contacts of said frame, a relay energized by rsaid Lsst source of voltage, `:and means controlled f' by 'fsaid relay to connect'rsaid secondrsourcefofvoltage lto the multiple :sleeve :contacts I of 'said frame.

2. 'In :a telephone asystem,1tl'1`e combination ofza. calling partys telephone set,"a called pantyseteleq phone set,4 a frame having Agroups of Atipfring and sleeve I contacts which rare.rniiltipledfto each other, thecal'ling partys telephone set being connected to :the multiple-tip'and -ringfcontacts cfrsaid frame, a ,rst-zsoinfceof voltagarafsecond source cf ,like

Voltage, impedances in series with saidfirst-and assunse ond :source of voltage 'to the 'multiple sleeve lcon 4tacts'foflsxazidframel 3. In a telephone "system,` the combination' of fa .callinggpartys telephone set, 1a pluralityv of -a'dditionalr telephone sets, a frame `having ,tip, ring and sleeve contacts which are multipled to veach other, rst and second ysources gof equalvoltage, meansresponsiveto the connection of said calling partys set to fone of the additional .telephone sets rto apply the voltage of said first source to :the multiple fsleeve contactso'f said framefa plurality of selector lcircuits each 'of Awhich is Aassociated with onefof'fsaid additional telephonewsets, said means including a relay operated by current'from said first source, said relay connecting the second sourcelto .the :multiplesleeve contacts vof `said frame,'said lsources having impedances in vseries therewith which are of substantially dilerent magnitude, la `recording circuit, and meansvresponn sivezto the simultaneous connection 'of ftheselectors .of other .ofy said additional telephone sets to the multiple fsleeve contacts of lsaid frame to operate said recording circuit.

HAROLD 'HENRY SCHROEPPEL 

